The overarching goal of this proposal is to identify and characterize the auditory processes that limit detection and discrimination of spectral cues for stimuli that vary in level, including both normal- hearing and hearing-impaired listeners. Aim 1 examines the effect of stimulus level fluctuation on spectral shape discrimination using the psychoacoustic paradigm of profile analysis. Level rove is often introduced in psychophysical tests of spectral shape discrimination in order to limit the use of absolute level cues, but preliminary data indicate that this manipulation has greater effect above than below 1 kHz. Such a result would have important implications for what we think we know about sensitivity for spectral shape. Aim 2 concerns the relationship between profile analysis and vowel perception, including potential effects of level fluctuation on vowel discrimination. Experiments in this aim will clarify the association between these two paradigms, often assumed to be closely related, and the extent to which psychophysical data on spectral shape discrimination inform our understanding of speech perception. Aim 3 explores the role of temporal fine-structure in masked speech perception, with emphasis on speech recognition in fluctuating noise. Whereas recent studies in the literature provide support for the idea that temporal fine-structure plays a key role in hearing in modulated noise, the proposed experiments introduce control conditions and test plausible alternative hypotheses that have not yet been rigorously evaluated. This issue is particularly pertinent given recent interest in developing cochlear implant technology that preserves fine timing cues. Aim 4 develops novel cuing paradigms that have the potential to provide new insights about our ability to extract signals from fluctuating backgrounds. One goal of this approach is to develop a greater understanding of the preconditions necessary for observing masking release. It is anticipated that the work proposed in these four aims will clarify the role of random level fluctuation in the processing of spectral cues present in natural sounds, such as those underlying speech recognition in background noise. PUBLIC HEALTH RELEVANCE: Level fluctuation can have a marked effect on auditory processing, degrading sensitivity in some conditions and improving it in others. The overarching goal of this proposal is to identify and characterize the auditory processes that limit detection and discrimination of stimuli that vary in level for normal-hearing, hearing-impaired, and cochlear-implanted listeners. Parallel experiments in psychoacoustics and speech perception paradigms are expected to result in greater understanding of basic auditory processes and as well as clinically relevant findings.